A typical envelope handling device includes an envelope feeding structure for feeding an envelope or a batch of envelopes in singular fashion in a downstream path of travel to a workstation. At the workstation a variety of operations can be performed on the envelope. For example, in a postage meter, a postal indicia is printed on the envelope. In other more generic printers, any desired image, barcode, address, etc., may be printed on the envelope. Another operation that can be performed is the insertion of documents into the envelope as in an inserter. After the operation is complete, the envelope is fed out of the workstation and ejected from the envelope handling device where it is collected in a drop stacker.
The purpose of the drop stacker is to collect and neatly stack the envelopes so that an operator may easily remove the envelopes from the drop stacker after processing a batch of envelopes. The drop stacker is generally used in low to medium volume operations. If a large volume of envelopes are being processed, then power stackers employing moving conveyor belts to carry away and collect the envelopes are typically used.
When processing a batch of envelopes, it is desirable to keep the envelopes separated. Specifically, the leading edge of an envelope should not run into the trailing edge of the previous envelope. Otherwise, misfeeds and jams occur which require operator intervention to correct. It is also desirable to achieve a high rate of throughput and thus process the batch of envelopes as quickly as possible. For these and other reasons, envelopes are typically fed out of the printing station faster than they are feed into the printing station. Thus ensuring that an incoming envelope will not collide with an exiting envelope.
Typical envelope handling devices employ ejection rollers or ejection belts operating at a constant speed. This constant speed is selected to be sufficiently high to avoid envelope collisions, but has been observed to cause other problems as discussed below.
The drop stacker includes an end wall, a registration wall and a stacking surface. The end wall is substantially vertical and generally perpendicular to the direction of envelope travel. It prevents the further downstream travel of the envelopes. The registration wall is substantially vertical and generally aligned to the direction of envelope travel. It serves to align the envelopes along their major length. The stacking surface is substantially horizontal and generally aligned below the exit plane of the thermal postage meter. It supports the envelopes as they accumulate in the drop stacker. As the envelope exits the thermal postage meter it continues on a slightly downward flight path, because of gravity, until it strikes the end wall at which point the envelope drops to the stacking surface. It has been observed that in typical ejection systems when the envelope strikes the end wall, the impact produces noise and also causes the envelope to bounce back away from the end wall. It has further been observed that the amount of noise and bounce back are proportional to the velocity of the envelope.
Both noise and bounce back are undesirable. Excessive noise tends to distract and irritate not only the machine operator but also others who are working in the surrounding area. Excessive bounce back prevents the envelopes from aligning neatly along their minor length. Therefore, there is a need to develop an ejection control system for an envelope handling device that reduces noise and bounce back in the drop stacker but still provides for high envelope throughput and avoids envelope collisions.